The present invention relates to a semiconductor laser device and a method for fabrication thereof, wherein the semiconductor laser device comprises a first and a second reflecting region, said first reflecting region being formed of a Bragg-mirror comprising alternating parallel layers having different refractive index, respectively, said first and second reflecting regions being adjacent to a laser active semiconductor region for emitting radiation of a predetermined wavelength and defining a longitudinal laser cavity, and exciting means for exciting said laser active semiconductor region to cause emission of radiation.
Semiconductor laser devices are steadily gaining in importance in a plurality of industrial applications. In particular, in the fields of gas spectroscopy, coupling of laser light into optical fibers, and in communication systems where a high transmission rate is required, semiconductor laser devices with high spectral purity, i.e. with single mode output radiation in the longitudinal as well as the transverse directions, are highly desirable. The type of surface-emitting laser devices having a vertical cavity (VCSEL) especially represents an important development, since the possibility of manufacturing a large plurality of such laser devices on a single semiconductor wafer in combination with control and drive circuitries for each single laser provides a laser device with high efficiency and low power consumption in conjunction with low manufacturing costs. Due to the small longitudinal extension of the laser-active region of a VCSEL (some hundreds of nanometer), the longitudinal radiation mode is a single mode one. The short longitudinal extension of the active region, in turn, requires highly reflective boundaries of the cavity or resonator mirrors in order to exceed the laser threshold. These highly reflective cavity boundaries are generated in that alternating semiconductor layers having a different index of refraction are formed with a thickness of a quarter-wavelength of the desired laser radiation. Such a stack of subsequent semiconductor layers having a different refractive index from one layer to an adjacent layer is known as a Bragg mirror. The reflectivity of such a Bragg mirror can achieve 99.9% and more depending on the number of pairs of alternating layers with a different refractive index.
The transverse extension of a cavity of a VCSEL, in general, is considerably larger than the longitudinal extension of the cavity, and hence, the laser beam emerging from a VCSEL usually comprises a plurality of transverse radiation modes. As a result, the output beam comprises a lower spectral purity and shows a deterioration of its beam profile. A variety of approaches is known in the art in order to provide VCSEL devices having a single mode radiation output.
For example, in IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 9, NO. 10, OCTOBER 1997, pp 1304-1306, a VCSEL is disclosed comprising one or more oxide layers formed by selective oxidation within the Bragg mirror. This oxide layer serves as current aperture as well as an optical aperture restricting the optical cavity in the transverse directions. By means of this oxidation layer, the lowest order transverse radiation mode may be selected. However, for incorporating the oxide layer in the Bragg-mirror, an additional manufacturing process step is required, resulting in increased cost and lower production yield. Moreover, due to the limited lifetime of laser devices having selectively oxidised layers, such laser devices are merely used in the fields of research and development, but not for industrial applications.
In APPLIED PHYSICAL LETTERS, VOL. 72, NO. 26, JUNE 1998, pp. 3425-3427, a VCSEL device is disclosed having a surface which is treated by means of etching procedures in order to generate a fine structure on top of the surface. This additional structure is formed so as to increase the optical losses of the radiation having transverse radiation modes of higher order, thereby providing selectivity and preferred amplification of the lowest order transverse mode. Again, these devices require an additional procedural step in manufacturing the device and this additional step of generating said etched structure demands high accuracy and control in both the transverse position and the depth of the structure.
In IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 6, NO. 3, FEBRUARY 1994, pp. 323-325, a VCSEL is described comprising a loss guided or xe2x80x9canti-guidedxe2x80x9d structure having in the area of the distributed Bragg reflector (DBR) aside of a Mesa structure a higher refractive index than in the DBR area below the Mesa. Thereby, increased optical losses of the radiation with higher order of the transverse radiation mode are achieved. The manufacturing of the anti-guided structure, however, requires a second epitaxial growth step resulting in a considerably increased production time and higher costs of such laser devices.
The VCSEL devices currently used in industrial applications comprise an ion-implanted region surrounding the laser active region of the VCSEL device, thereby restricting said laser active region. Due to ion-scattering in the crystalline material, the distribution of the implanted ions is difficult to control, resulting in a rather diffuse implantation region. Accordingly, the transverse extension of the laser active region is defined at low precision. As a further consequence, since small transverse dimensions of the laser active region which are necessary for a single mode laser operation are not easily achievable, additional measures for the transverse confinement of the laser beam have to be taken while manufacturing the above-mentioned VCSEL devices, thereby causing increased production cost.
It is therefore an object of the present invention to provide a semiconductor laser device as described in the preamble of claim 1 having an improved mode selectivity avoiding any of the afore-mentioned disadvantages of the prior art, and a method for fabricating the same.
In a first aspect, the above-mentioned object is solved by a semiconductor laser device according to the present invention, comprising a first and a second reflecting region, said first reflecting region being comprising a Bragg mirror with alternating parallel layers having different refractive index, respectively, said first and second reflecting regions being adjacent to a laser active semiconductor region for emitting radiation of a predetermined wavelength and defining a longitudinal laser cavity; exciting means for exciting said laser active semiconductor region causing emission of radiation, wherein said first reflecting region further comprises a contact layer on top thereof, and one additional layer on top of said contact layer covering only a portion of said contact layer, wherein an arrangement of said alternating layers forming said Bragg mirror, an optical thickness of said contact layer, and a reflectivity and an absorption of said additional layer is selected so as to reduce reflectivity of said reflecting region in said portion of said contact layer covered by said additional layer, resulting in a smaller reflectivity compared to a portion of said contact layer, which is not covered by said additional layer.
In a second aspect, a method for fabrication of a semiconductor laser device is provided, wherein the semiconductor device comprises at least one Bragg mirror for forming an optical laser cavity, said method comprising the steps of: forming a stack of layers alternatingly having a different index of refraction for forming said Bragg mirror on top of a laser active semiconductor region; forming a contact layer on top of said layers of said Bragg mirror; and forming a metallization layer on top of said contact layer, said metallization layer consisting of at least one layer of metal having a defined thickness, wherein said metallization layer is formed so as to cover only a portion of said contact layer and a portion not covered by said metallization layer forms a radiation emission window; said method further including a calculation step for determining a required optical thickness of said contact layer and/or an optical thickness of said metallization layer required to achieve a difference in reflectivity of said Bragg mirror id a region where said metallization layer covers a portion of said contact layer compared to a region where said metallization layer does not cover said contact layer.
Regarding the first aspect of the present invention, in the semiconductor laser device of the present invention, a portion of the reflecting region where the higher transverse radiation modes would be reflected, is covered by the additional layer with high optical density thereof, and an underlying contact layer sandwiched between the Bragg mirror and the additional layer is provided, whose thickness is optimised to minimise the reflectance of said portion covered by the contact layer and the additional layer for a predetermined wavelength, while the reflectance of the remaining area of the reflecting region not covered by the additional layer remains substantially not affected by the contact layer. Advantageously, the required accuracy of the thickness of the contact layer is considerably lower than the required accuracy of the layers forming the Bragg reflector, and, hence, the industrial production of semiconductor laser devices having the aforementioned contact layer with optimised thickness is uncritical.
Preferably, the thickness of the contact layer is selected such that a difference in reflectivities of the reflecting region not being covered by the additional layer and the reflecting region covered by the additional layer, respectively, is maximised.
It should be noted in this context that the term xe2x80x9cminimum reflectivityxe2x80x9d used in claims 2 and 4 not only includes the exact minimum reflectivity which can be obtained with a specific arrangement of the Bragg mirror, the additional layer and the contact layer, but also includes larger reflectivities leading to a difference in reflectivities of more than 75% of the maximum difference in reflectivities that is achieved with the exact minimum reflectivity. For such larger reflectivities the advantage of the present invention can still be attained. In other words, slight deviations in the reduced reflectivity are still satisfactory to achieve the desired effect, and, therefore, should be considered as included in the said claims.
Advantageously, the semiconductor laser device according to the present invention is formed such that said portion not covered by said additional layer serves as a radiation emission window for said radiation. In this way, the edges of the additional layer are used as an optical aperture.
In a preferred embodiment of the present invention, the extension and position of said radiation emission window are adapted to substantially select a predetermined single transverse radiation mode of said radiation.
In a preferred embodiment the additional layer comprises material suitable for providing contact metallization of the VCSEL device such as a metal or other electrical conductive material. Thus, an element of the device necessary for its operation is simultaneously used for mode selection. Preferably the additional layer is a compound metal comprising Ti, Pt, and Au.
Preferably, the aforementioned additional layer comprising a metal serves as electrode. Thus, since the step of forming electrodes has to be performed at any rate, the semiconductor laser device according to the present invention comprising means for selecting a single mode radiation can be manufactured without an additional step compared to the prior art semiconductor laser devices.
Preferably, the predetermined single transverse radiation mode is the lowest order transverse radiation mode.
In a preferred embodiment of the present invention, the number of pairs of said alternating layers having different refractive index forming said first reflecting region is selected so as to obtain the minimal absolute reflectivity required to exceed the laser threshold. By this measure, a maximum difference of reflectivity of the aforementioned regions and the effective reduction of undesired transverse radiation modes is increased. Moreover, a lower number of alternating layers involves a lower number of manufacturing steps, resulting in lower cost and higher yield.
In a preferred embodiment of the present invention, the semiconductor laser device is a vertical cavity surface emitting laser.
Further preferred embodiments are defined in the dependent claims.
Regarding the second aspect of the present invention, an advantage of the method described therein is that it can be carried out without adding an additional procedural step while manufacturing the semiconductor laser device, thereby decreasing production cost and increasing production yield. Moreover, the method of the present invention may be applied in combination with all known methods for forming a semiconductor laser being capable to emit a single mode radiation.